The accurate way is to download the board manual and check the layout diagram. I checked out a couple of the Supermicro (X9SCM/X9SCL) and Tyan (S5512/S5510) manuals and I didn't see a switch on the diagram.By the way, while searching & filtering w/ pci-e slots quantity : how do i distinguish deal breaker mobos (all native pci-e slots) from the ones using pci-e switches ?
The quick (and unreliable) way is to just add up the lane count. You'll see that a few have 2 x8 and 2 x4 lanes (note: lane size, not slots size). Anything less than around 24 lanes should be alright.
I should clarify my earlier statement as I didn't check all board manuals - it doesn't appear that any boards are using a PCIE switch given the lane count.